Commonly used iron oxide type fine particles and ferromagnetic metal particles which are used for improving a magnetic recording density and a reproduced output because of their high saturation magnetization and high coercive force have been investigated as ferromagnetic particles.
The dispersibility of ferromagnetic particles has continuously been improved in order to improve the characteristics of a magnetic recording medium, particularly the electromagnetic properties such as the sensitivities of S/N ratio. However, ferromagnetic metal particles have problems in that the particles readily aggregate because of their high magnetization. Thus, a coating composition having the particles well dispersed therein cannot be obtained. As a result, a smooth magnetic layer having a good surface property cannot be obtained.
Additionally, ferromagnetic metal particlas are easily oxidized and are unstable in air. A magnetic tape (a metal tape) prepared using ferromagnetic metal particles thus has a tendency that the magnetization is reduced (demagnetization) particulary when the tape is exposed to high humidity.
It is necessary that the particle size of the ferromagnetic metal particles is minimized and that the saturation magnetization (.sigma.s) is increased in order to improve the sensitivity and S/N of a magnetic recording medium (or a magnetic metal tape) using ferromagnetic metal particles.
However, as the particle size is minimized and as saturation magnetization is increased, the dispersibility and stability of the magnetic characteristics deteriorates.
Various methods for providing ferromagnetic particles with a surface treatment have been proposed in order to improve the dispersibility of the ferromagnetic particles and to improve the demagnetization of the ferromagnetic metal particles are described, for example, in U.S. Pat. Nos. 3,700,499, 3,284,358, 4,420,330 and 4,369,076.
One of the methods which is commonly employed is forming an oxide layer on the surface of the ferromagnetic metal particles. However, this does not provide satisfactory results. Further, a chemical treatment and a method for chemically connecting organic compounds such as a complex and a coupling agent on the surface of the ferromagnetic metal particles have also been proposed.
Of the proposals, the method for chemically connecting organic compounds on the surface of the particles is the most efficient because the caking phenomenon hardly occurs among the particles at the time of treatment and the treating agent does not separate from the surface of particles. Further, the dispersibility of particles is greatly improved at the time of mixing a binder in the coating composition. However, the surface property of a magnetic layer which is coated and dried is not greatly improved and on the contrary, it often happens that the surface property thereof is worse than that of a magnetic layer comprising particles which have not been subjected to surface treatment.
The above problem is more serious as the amount of an organic compound which connects on the surface of particles increases.
A certain amount of an organic compound is necessary in order to prevent ferromagnetic metal particles from demagnetization. But this method is not satisfactory because of the above described problem.